Call me crazy, but my fiance has been watching a lot of UFO stuff lately. I am extremely skeptical, but it put a question into my head, that I can not seem to find an answer to in my random searches on google. Can an object that makes no sound while accelerating create a sonic boom when breaking the sound barrier? Whenever I read about the sound barrier I find lots of answers on physics theory, calculations, examples, pictures, etc. All things that I have found refer to aircraft breaking the speed of sound. Well we all know how loud planes are, and I can understand the loud sound when a sound is generated by the aircraft itself. But what about, theoretically, a propulsion system that is completely silent? Any help?